A fuel cell is a galvanic device which operates in accordance with similar electrochemical principles as in conventional storage batteries, i.e., a positive and negative electrode are separated by an ion-conducting electrolyte adapted to carry current generated by a catalyzed chemical reaction. Unlike the storage battery, however, the fuel cell has a theoretically infinite energy output capacity, dependent solely upon the continuous supply of fuel and oxidant to the reaction system. For the traditional hydrogen-oxidant fuel cell, current flow is provided by the flow of electrons associated with the passage of a positive hydrogen ion through an intervening electrolyte medium to the cathode in the oxygen-containing chamber of each cell, resulting in the formation of water and the generation of electric current. The energy value of the thus generated current, i.e., the potential energy of the voltage drop, is directly proportional to the energy output resulting from the exothermic oxidation reaction of hydrogen and the oxygen (in the air) to form water. Fuel cells, generally, have one common limitation: the voltage output of a cell decreases with increasing current flow, or amperage, i.e., as the power being drawn from the cell increases.
In the past, there have generally been three distinct types of hydrogen-oxygen fuel cells which are capable of operating at temperatures below about 500.degree. F.: the solid polymer proton exchange membrane fuel cell; the alkaline fuel cell; and the phosphoric acid fuel cell. Each of these types are generally well known and further description is not needed for this invention. In general, however, it is the solid polymer proton exchange membrane fuel cell which will be most effective in the present invention. In these preferred fuel cells, electrical energy is produced by the catalyzed reaction between hydrogen and an oxidizing gas, generally oxygen, either pure or diluted, as in air.
In the first type above, a solid polymeric membrane, capable of passing ions, such as the hydrogen ion, or proton, and molecular water, but not the hydrogen or oxygen gases, is used to separate the two gases in anode and cathode chambers, respectively. These membranes have been formed from, for example, a sulfonated fluorocarbon polymer sold, for example, under the trademark "Nafion".RTM. by E.I. Dupont de Nemours, or a more recently developed polymer by the Dow Chemical Corporation. Other suitable materials can be used together with this invention, but which do not form a part of this. A noble metal catalyst, for example platinum, in small quantities, is generally embedded in or located in direct contact with, the polymeric membrane. These catalytic metals also act as the electrodes for the cell. Bipolar current collectors/separators are used to separate adjacent cells and as the means of collecting and transmitting current flow to outside of each cell. These cells are maintained in "stacks" of a plurality of individual power cells in a series, e.g., 20 cells, which then generate the total voltage output. As a general rule, the voltage generated by a power cell varies inversely with the output amperage taken by the load.
The fuel cell stacks, which generate water as a by product from the chemical reaction of hydrogen-containing fuel and an oxygen-containing oxidant, also require the use of water for cooling and for maintaining the integrity of the electrolyte membrane. In many cases, the by-product water is sufficient to maintain the cooling of the system and to provide the needed humidification of the incoming reactant gases to maintain the integrity of the membrane during operation of the fuel cell. Water is carried from the fuel, or hydrogen, side of the membrane, together with the proton, through the membrane and thus tending to dry the anode side of the membrane, ultimately causing cracking of the membrane if additional water is not provided to compensate for such loss.
Generally, it is known to pre-humidify the system by passing the fuel and oxidant gases through humidification cells within the cell stack, or to externally humidify the fuel and oxidant gases. Examples of such systems are shown in commonly owned U.S. Pat. No. 5,047,298, and in an earlier U.S. Pat. No. 4,214,969. With internal humidification, the fuel and oxidant gases are initially passed independently through humidification cells within the cell stack. The gases are there saturated, or almost saturated, with water vapor and then passed, also in parallel, through the individual power cells within the stack.
It has also been previously suggested that in order to most efficiently use a fuel cell stack for vehicular propulsion, the stack should be, preferably, sized so as to provide sufficient power, at a useful voltage, for normal continuing operation, or cruising operation, when utilizing air as the oxidant, and that during peak loads, pure oxygen should be substituted for air as the oxidant. This allows the fuel cell stack to be sized for normal low power/air operation, but also to provide a peak power capacity, at a suitable voltage, significantly greater than for normal operation, and without any complex changes to the system. Such a system is shown in U.S. Pat. No. 4,657,829. In this prior patent, the water generated by operation of the fuel cell is electrolyzed during normal operation by the excess electrical capacity of the fuel cell. The electrolysis results in the generation of hydrogen and oxygen gases, which in turn are stored under pressure for use when required at peak power capacity. Although this system does result in the desired peak power availability, the amount of oxygen which must be stored in order to have adequate peak power capacity is a problem for a vehicle for which minimum design weight is desired.
It is thus an object of the present invention to provide a fuel cell power system for a vehicle with improved peak power capability but with minimized high pressure gas storage requirement. It is yet a further object of the present invention to provide a fuel cell power system utilizing power created during operation of the vehicle and water generated by operation of the fuel cell to generate oxygen and hydrogen for use during peak power intervals, but wherein the effectiveness of the oxidant air is enhanced by enrichment with oxygen so as to reduce the amount of storage capacity required for peak acceleration requirements. It is yet another and further objective of the present invention to provide a fuel cell powered vehicle having improved efficacy during operation. Other objects and advantages will become apparent when considering the following specific description of an example of the invention.